If your vehicle is showing a/c blows warm or hot air, here are the most likely causes and the parts you'll need to fix each one.
The system has lost refrigerant through a small leak in a seal, hose, or the condenser. Without enough refrigerant, the system cannot absorb and remove heat from the cabin.
Adding too much 'freon' from a DIY auto parts store can cause system pressure to spike. The pressure safety switch will then disable the compressor to prevent catastrophic damage.
The climate control system uses a small motor to mix hot and cold air. If this motor fails or gets stuck, it may continuously blow hot engine coolant heat into the cabin regardless of the A/C setting.
The A/C condenser relies on a cooling fan to pull heat away from the refrigerant. If the fan fails, the A/C will blow warm air when the vehicle is stopped or idling, but may get cold again at highway speeds.
The compressor clutch engages the A/C compressor to the engine's belt. If the clutch wears out, or if a relay or fuse blows, the compressor will never turn on to pump refrigerant.
This tiny valve regulates the flow of cold refrigerant into the cabin. If it becomes clogged with debris or moisture, it starves the evaporator, resulting in warm air.
The A/C compressor has internally failed or seized, sending metal debris throughout the system. This requires replacing the compressor and flushing or replacing most other A/C components.